1. Technical Field
This invention relates to an illumination device which includes a detector for producing a signal indicative of the light output of the device and which signal is independent of light incident on the device from the exterior of the device.
2. Background Art
U.S. patent application Ser. No. 742,358, filed June 7, 1985 in the name of D. L. Funston describes illumination control circuitry for a film video player electronic strobe light. The film video player therein described includes an electronic strobe light operable for repetitively firing a flash tube to flash illuminate a solid state image sensor via a pulse of light projected onto a photographic film at the vertical retrace rate of a standard television signal. Detector circuitry, responsive to a signal relating to an output of the image sensor, produces a control signal that is functionally related to whether or not the illumination of the image sensor is above, below, or within a predetermined range near the saturation level of the image sensor. A microprocessor adjusts an exposure level at the vertical retrace rate in accordance with the control signal. Logic switching circuitry, having an input coupled to the microprocessor and responsive to the exposure level signal and an output coupled to the electronic strobe light, causes the electronic strobe light to produce a pulse of light within each vertical retrace interval that is sufficient to illuminate the image sensor near its saturation level without the illumination saturating the sensor, to optimize the signal-to-noise ratio of the image sensor.
The light source described in the aforesaid prior application includes a Xenon flash tube positioned adjacent a reflector, arranged for directing light from the flash tube onto a light integrator, which is formed of a solid bar of transparent material. A diffuser at the output of the integrator functions to spread the light so that there are no illumination discontinuities over the film plane, as such discontinuities may result in either light or dark "bands" being visible on the video monitor. A photosensor, such as a photodiode, produces a light-dependent signal corresponding to the intensity of light from the flash tube. For that purpose, the photosensor is positioned to receive light reflected from the surface of the diffuser. Because light reflected from the film varies with the density of the negative, the photosensor is located, or its viewing angle is restricted, so that light reflected from the film does not impinge on the photosensor.